The present invention relates, in general, to a broad class of transducers which are herein referred to as "electroacoustical" transducers. These are transducers such as (a) speakers which are driven by an electrical signal and produce an acoustic output or sound, or (b) microphones which are activated by sound pressure signals and produce an electrical output.
Transducers of this type are known to have an inherent dependency upon, or sensitivity to, the frequency of the signal which drives the transducer. In my U.S. Pat. No. 3,988,541, issued Oct. 26, 1976, a method and apparatus is disclosed for compensating for the frequency dependency of the mechanical response parameters of the transducer. As defined there, the mechanical response parameters include the mass, M, of the cone and transducer coil, the elasticity or spring parameter, K, in the suspension system for the cone and coil, and the loss, D, including the dissipation in the cone and the work performed in vibrating the air adjacent the cone to generate the acoustic output. Thus, the three parameters of mass, spring parameter and loss are referred to as the "mechanical response parameters" of the transducer.
Briefly, the referenced patent provides a current controlled power amplifier to drive the speaker; and the input signal of this current amplifier is modified to have a frequency characteristic which is the complement of the corresponding frequency characteristic of the mechanical response parameters of the transducer. In this context, the term "complement" is intended to mean that the input signal to the power amplifier is made large where the mechanical response is small and vice verse. In other words, for a given primary input signal of constant magnitude over the desired frequency range, where a relatively large output signal is produced by the mechanical parameters, the signal processing network acts to attenuate the primary input signal. For those frequencies at which the output of the transducer is relatively small, then the signal processing network reinforces or strengthens the driving signal such that the compensation network modifies the primary input electrical signal in such a manner that the resulting signal fed to the speaker has a frequency characteristic which is the complement of the corresponding frequency characteristic of the mechanical response parameters of the speaker. The result is that the velocity of the diaphragm or cone is substantially uniform over the frequency range of interest.
I have found that in addition to the frequency dependency caused by the mechanical response parameters of a speaker, there are other phenomena which are frequency dependent, and therefore cause a frequency dependency in the overall acoustic response of the transducer, at least over a portion of the range. Further, I have discovered that these phenomena can be isolated and independently compensated.
The principal frequency dependent characteristics of a transducer not including the environment in which it is located comprise: (1) the effect of the mechanical response parameters of the transducer, as discussed above; (2) the effect of the cone-air power transfer characteristic; (3) the effect of cone break-up. The latter effect is not always present, but if it is detected, compensation can be made according to the present invention. Further, the present invention permits compensation for the effects of room acoustics.
To compensate for the first three effects, the speaker is placed in an anechoic chamber, and its audio response is measured over a desired frequency range using a calibrated microphone to measure sound pressure levels. The composite anechoic audio response is separated into three distinct characteristic curves, namely (1) the mechanical response characteristic of the speaker; (2) the cone-air power transfer characteristic; and (3) the cone break-up characteristic. Cone break-up does not occur in all speakers, but it is a phenomenon in which all portions of the speaker diaphragm do not vibrate in unison. In other words, standing waves may exist usually along the radial direction of the cone, and since some of the harmonics are out of phase with others and with the fundamental, there is a power loss in the acoustic output.
Compensation is made for each of these phenomena by means of a network having a frequency characteristic curve complementary to the frequency characteristic of the associated phenomena. When the compensation networks are all connected in tandem, the system processes the incoming signal to achieve substantially uniform audio response over the desired frequeny range. By uniform audio response it is meant that the sound pressure level measured in an anechoic chamber is substantially uniform- i.e., within a few decibels, as distinguished from the variations of orders of magnitude without compensation. After compensation is made for these three phenomena, the compensated speaker may be placed in a room and the response measured at a given point in the room. Still another compensation network may be added to process the input signal to the speaker in a fashion similar to the compensations made for the mechanical response characteristics of the speaker and the cone-air transfer characteristic. In some rooms the sound-transmission characteristics may exhibit such a large number of reflections and reverberations that this compensation may not be worthwhile, but the principles of the present invention will enable compensation to be made for most cases.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of an illustrative embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like elements in the various views.